Mice provide a well-studied homogeneous population that can be manipulated readily for experiments examining the effect of age on the immune system. By using murine models to study aging, we can easily examine different components of the immune system. Furthermore, the availability of genetically manipulated mouse models will allow us to study the effect of age on a single parameter at a time. This Core Facility will provide services not currently provided by the Trudeau Institute animal facility. The specific aims of Core B will be: (1) generation of a colony of healthy aged mice - the Core will ensure that aged mice are maintained on ventilated racks and that monthly health screenings are performed; (2) generation of new inbred strains of genetically altered mice - the Core will backcross knockout and transgenic mice onto appropriate genetic backgrounds, as needed by each investigator. This Core will also ensure that all mice needed by the investigators in the Program are screened in a timely manner. These mice can then be purchased by each investigator.